Not Applicable
The present invention is directed generally to the transmission of information in communication systems. More particularly, the invention relates to transmitting information via optical signals in optical transmission systems and transmitters for use therein.
The development of digital technology provided resources to store and process vast amounts of information. While this development greatly increased information processing capabilities, it was soon recognized that in order to make effective use of information resources, it was necessary to interconnect and allow communication between information resources. Efficient access to information resources requires the continued development of information transmission systems to facilitate the sharing of information between resources.
The continued advances in information storage and processing technology has fueled a corresponding advance in information transmission technology. Information transmission technology is directed toward providing high speed, high capacity connections between information resources. One effort to achieve higher transmission capacities has focused on the development of optical transmission systems for use in conjunction with high speed electronic transmission systems. Optical transmission systems employ optical fiber networks to provide high capacity, low error rate transmission of information over long distances at a relatively low cost.
The transmission of information over fiber optic networks is performed by imparting the information in some manner to a lightwave carrier by varying the characteristics of the lightwave. The lightwave is launched into the optical fiber in the network to a receiver at a destination for the information. At the receiver, a photodetector is used to detect the lightwave variations and convert the information carried by the variations into electrical form.
In most optical transmission systems, the information is imparted by using the information data stream to either modulate a lightwave source to produce a modulated lightwave or to modulate the lightwave after it is emitted from the light source. The former modulation technique is known as xe2x80x9cdirect modulationxe2x80x9d, whereas the latter is known as xe2x80x9cexternal modulationxe2x80x9d, i.e., external to the lightwave source. External modulation is more often used for higher speed transmission systems, because the high speed direct modulation of a source often causes undesirable variations in the wavelength of the source. The wavelength variations, known as chirp, can result in transmission and detection errors in an optical system.
Data streams can be modulated onto the lightwave using a number of different schemes. The two most common schemes are return to zero (RZ) and non-return to zero (NRZ). In RZ modulation, the modulation of each bit of information begins and ends at the same modulation level, i.e., zero, as shown in FIG. 1(a). In NRZ schemes, the modulation level is not returned to a base modulation level, i.e., zero, at the end of a bit, but is directly adjusted to a level necessary to modulate the next information bit as shown in FIG. 1(b). Other modulation schemes, such as duobinary and PSK, encode the data in a waveform, such as in FIG. 1(c), prior to modulation onto a carrier.
In many systems, the information data stream is modulated onto the lightwave at a carrier wavelength, xcexc, (FIG. 2(a)) to produce an optical signal carrying data at the carrier wavelength, similar to that shown in FIG. 2(b). The modulation of the carrier wavelength also produces symmetric lobes, or sidebands, that broaden the overall bandwidth of the optical signal. The bandwidth of an optical signal determines how closely spaced successive optical signals can be spaced within a range of wavelengths.
Alternatively, the information can be modulated onto a wavelength proximate to the carrier wavelength using subcarrier modulation (xe2x80x9cSCMxe2x80x9d). SCM techniques, such as those described in U.S. Pat. Nos. 4,989,200, 5,432,632, and 5,596,436, generally produce a modulated optical signal in the form of two mirror image sidebands at wavelengths symmetrically disposed around the carrier wavelength. Generally, only one of the mirror images is required to carry the signal and the other image is a source of signal noise that also consumes wavelength bandwidth that would normally be available to carry information. Similarly, the carrier wavelength, which does not carry the information, can be a source of noise that interferes with the subcarrier signal. Modified SCM techniques have been developed to eliminate one of the mirror images and the carrier wavelength, such as described in U.S. Pat. Nos. 5,101,450 and 5,301,058.
Initially, single wavelength lightwave carriers were spatially separated by placing each carrier on a different fiber to provide space division multiplexing (xe2x80x9cSDMxe2x80x9d) of the information in optical systems. As the demand for capacity grew, increasing numbers of information data streams were spaced in time, or time division multiplexed (xe2x80x9cTDMxe2x80x9d), on the single wavelength carrier in the SDM system as a means to provide additional capacity. The continued growth in transmission capacity has spawned the transmission of multiple wavelength carriers on a single fiber using wavelength division multiplexing (xe2x80x9cWDMxe2x80x9d). In WDM systems, further increases in transmission capacity can be achieved not only by increasing the transmission rate of the information via each wavelength, but also by increasing the number of wavelengths, or channel count, in the system.
There are two general options for increasing the channel count in WDM systems. The first option is to widen the transmission bandwidth to add more channels at current channel spacings. The second option is to decrease the spacing between the channels to provide a greater number of channels within a given transmission bandwidth. The first option currently provides only limited benefit, because most optical systems use erbium doped fiber amplifiers (xe2x80x9cEDFAsxe2x80x9d) to amplify the optical signal during transmission. EDFAs have a limited bandwidth of operation and suffer from non-linear amplifier characteristics within the bandwidth. Difficulties with the second option include controlling optical sources that are closely spaced to prevent interference from wavelength drift and nonlinear interactions between the signals.
A further difficulty in WDM systems is that chromatic dispersion, which results from differences in the speed at which different wavelengths travel in optical fiber, can also degrade the optical signal. Chromatic dispersion is generally controlled in a system using one or more of three techniques. One technique to offset the dispersion of the different wavelengths in the transmission fiber through the use of optical components such as Bragg gratings or arrayed waveguides that vary the relative optical paths of the wavelengths. Another technique is intersperse different types of fibers that have opposite dispersion characteristics to that of the transmission fiber. A third technique is to attempt to offset the dispersion by prechirping the frequency or modulating the phase of the laser or lightwave in addition to modulating the data onto the lightwave. For example, see U.S. Pat. Nos. 5,555,118, 5,778,128, 5,781,673 or 5,787,211. These techniques require that additional components be added to the system and/or the use of specialty optical fiber that has to be specifically tailored to each length of transmission fiber in the system.
New fiber designs have been developed that substantially reduce the chromatic dispersion of WDM signals during transmission in the 1550 nm wavelength range. However, the decreased dispersion of the optical signal allows for increased nonlinear interaction, such as four wave mixing, to occur between the wavelengths that increases signal degradation. The effect of lower dispersion on nonlinear signal degradation becomes more pronounced at increased bit transmission rates.
The many difficulties associated with increasing the number of wavelength channels in WDM systems, as well as increasing the transmission bit rate have slowed the continued advance in communications transmission capacity. In view of these difficulties, there is a clear need for transmission techniques and systems that provide for higher capacity, longer distance optical communication systems.
Apparatuses and methods of the present invention address the above need by providing optical communication systems that include transmitters that can provide for pluralities of information carrying wavelengths per optical transmission source, dispersion compensation, and/or nonlinear management in the system. In an embodiment, the information data stream is electrically distorted to compensate for chromatic dispersion of a lightwave/optical signal during transmission. The electrical distortion can be used to compensate for negative or positive dispersion in varying amounts depending upon the characteristics of the optical fiber in the network and to some extent offset nonlinear interactions that produce distortion of the optical signal Electrical distortion can be specifically tailored to each wavelength and bit rate used in the optical system.
Electrical dispersion compensation can be used in conjunction with other methods, such as dispersion compensating fiber or time delay components to control the level of dispersion at various points in the network. The amount of dispersion in the system can be controlled to provide a substantially predetermined value of net dispersion, e.g., zero, at the end of a link, to provide an average value over the link, and/or to minimize the absolute dispersion at any point in the link.
Electrical distortion compensation can be used with RZ, NRZ, ASK, PSK, and duobinary formats, as well as other modulation formats and baseband and subcarrier modulation techniques. In addition, the amount of electronic distortion applied to a signal can be controlled via a feedback loop from a receiver in the system to allow fine tuning of the compensation. In this manner, changes in the network performance with time can be accommodated.
In an embodiment, an information data stream is modulated on to an electrical carrier, such radio frequency (xe2x80x9cRFxe2x80x9d) or microwave carrier, frequency xcexde. The modulated electrical carrier is upconverted on to a lightwave carrier having a wavelength xcex0 and frequency xcexdo produced by the optical transmission source to produce an information carrying lightwave at wavelength xcex1 and frequency xcexdoxc2x1e. The upconverter can be used to simultaneously upconvert a plurality of electrical frequencies onto different subcarrier lightwaves. In an embodiment, the information is modulated onto the electrical carrier in duobinary format, which provides for more narrow subcarrier bandwidths.
In an embodiment, the lightwave carrier from the optical source is split into a plurality of split lightwave carriers, each of which has one or more data streams upconverted or modulated onto it. The subcarrier lightwave optical signals generated from the split lightwave optical carriers are then recombined into the optical signal for transmission. The split lightwave carrier overcomes the problem of maintaining close wavelength spacing between multiple carriers in an operating system by employing a common optical source. The optical source providing the lightwave carrier may include one or more lasers or other optical sources.
The split lightwave carrier also provides a method of placing multiple information carrying wavelengths near the lightwave carrier without having to upconvert or modulate more than one data stream at a time onto a lightwave carrier. The upconverted lightwaves can be at wavelengths that are greater and/or less than the carrier wavelength and symmetrically or asymmetrically positioned relative to the carrier wavelength. In addition, subcarriers can be simultaneously unconverted onto the same lightwave, at least one subcarrier with a higher frequency and at least one subcarrier with a lower frequency than the carrier frequency.
The upconversion of the modulated electrical carrier can be performed using double or single sideband upconverters with or without suppression of the carrier wavelength xcexo. However, the reduction or elimination of the carrier wavelength xcexo and any mirror image sideband will eliminate unwanted signals which could interfere with the upconverted signal.
In an embodiment, a two sided, single sideband upconverter is provided to modulate multiple information data streams onto both longer and shorter wavelengths. In those embodiments, one upconverter can be used to upconvert data on equally or differently spaced subcarriers relative to the carrier wavelength.
In an embodiment, the polarization of adjacent lightwave carriers is controlled to decrease the nonlinear interactions of the signals. For example, adjacent wavelength signal can be orthogonally polarized to decrease the extent of four wave mixing that occurs between the signals during transmission. In addition, the wavelength spacing between neighboring upconverted signals can be selected to lessen non-linear interaction effects.
Accordingly, the present invention addresses the aforementioned problems with providing increasing the number of channels and the transmission performance of optical systems. These advantages and others will become apparent from the following detailed description.